EP0446900B1

EP0446900B1 - Mixed-flow compressor

Info

Publication number: EP0446900B1
Application number: EP91103850A
Authority: EP
Inventors: Hideo Nishida; Hiromi Kobayashi; Haruo Miura; Takeo Takagi
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1990-03-14
Filing date: 1991-03-13
Publication date: 1994-12-28
Anticipated expiration: 2011-03-13
Also published as: DE69106179D1; DE69106179T2; EP0446900A1; JPH03264796A; JPH07103874B2

Description

The present invention relates to a mixed-flow compressor comprising a mixed-flow impeller, in which the flow path in the meridional plane of the impeller is slanted relative to a radial direction, and a pair of diffuser plates provided downstream of the mixed-flow impeller, such that the inlet portion of the flow path through said diffuser plates is substantially aligned with the outlet flow path of said mixed-flow impeller and the outlet portion of the flow path through said diffuser plates is directed in a radial direction, and guide vanes arranged on one of the diffuser plates circumferentially around the rotary axis of the impeller in a circular row.
In the JP-Y-56 38 240, a mixed-flow compressor is described having guide vanes which are arranged on the hub plate.
In Proceedings of the Sixth Turbomachinery Symposium, pp 61 to 62 (October 1977), a conventional mixed-flow compressor is disclosed having an oblique or mixed flow diffuser (in which a flow-out direction is slanted from the radial direction) which is not provided with guide vanes in the flow path. In addition, it is known from the Journal of the Japan Society of Mechanical Engineering, pp 16 to 20 (March 1987) that a diffuser without guide vanes is provided in the radial direction.
The GB 693 727 discloses a mixed-flow compressor comprising guide vanes which are attached to the hub and the shroud, the inner diameter of the guides being larger than the outer diameter of the impeller.
In general, if the specific speed ns expressed in the following equation is high, an impeller inlet tip to outlet tip diameter ratio thereof is increased so that the performance of the impeller becomes low due to the fact that the curvature of the meridional flow path is increased in case of a centrifugal impeller. Further, due to the increase in curvature, a secondary flow becomes remarkable within the impeller, so that the flow at the outlet of the impeller is deflected to the hub side to thereby lower the performance of the diffuser.

where N is the rotational speed (rpm), Q is the volume flow rate (m³/min) and Had is the adiabatic head (m).
In order to avoid this problem, in general, a mixed-flow impeller is used in which the outlet of the impeller is slanted from the radial direction. In the mixed-flow impeller, the curvature of a flow path in a meridional plane (i.e., a cross section including the rotary shaft center) (hereinafter referred simply to as a meridional flow path) is decreased, so that the flow may be kept substantially uniform in the widthwise direction at the outlet of the impeller, i.e., at the inlet of the diffuser. It is thus possible to prevent the generation of the flow deflected toward the hub. However, if the flow having a tangential component enters into the mixed-flow diffuser, the flow is deflected to the shroud side from the intermediate portion to the outlet portion of the diffuser due to the curvature perpendicular to the flow path. In the extreme case, a reverse flow will be generated on the hub side to largely increase the diffuser pressure loss. In addition, a mixed-flow compressor comprising a mixed flow diffuser without vanes has an increased length in the axial direction. Therefore, the compressor is large in size and the friction loss in the flow path is high.
Furthermore, since the length of the rotary shaft is increased, the critical speed of the rotor must be lowered.
In order to overcome these problems, guide vanes each having a height corresponding to 10 to 50% of the width of the meridional flow path are provided on the hub side of the diffuser. However, this could not sufficiently attain the object. There are still unsolved problems such as the increase in friction loss and a reduction in critical speed.
The problem underlying the invention is to provide a mixed-flow compressor that is small in size and ensures a high performance.
Starting out from the mixed-flow compressor of the generic kind, this problem is solved by the characterizing features of claim 1.
Preferred embodiments of the mixed-flow compressor are included in claims 2 to 4.
With such arrangements, the following advantages are ensured.
Since the guide vanes, the height of which being smaller than the width of the flow path, are provided at the curved portion on the flow path surface of the diffuser plate on the shroud side curved in the radial direction in the vicinity of the outlet of the mixed-flow impeller, and the inlet and outlet angles are equal to the impeller outlet average flow angle at the design point, the flow on the shroud side of the mixed-flow impeller outlet is introduced with almost no shock into the guide vanes. Then, since the fluid introduced into the guide vanes is forcibly led, the fluid will flow without separating away from the wall surface of the shroud side and will reach the guide vane outlet portion. Since the vane angle is equal to the average flow angle at the design point at the guide vane outlet, i.e., the end of the curved portion, the flow angle of the fluid led by the guide vane is equal to the flow angle of the fluid at the portion where guide vanes are not provided. Also, since at the end of the curved portion, the curvature of the meridional plane flow path is small, the meridional flow velocity is also kept substantially uniform in the widthwise direction. After all, the flow may be kept uniform in the widthwise direction.
Also, since the guide vanes, the height of which is smaller than the width of the flow path, are provided on the surface of the flow path of the diffuser plate on the shroud side bent to the radial direction immediately after the impeller outlet, and the inlet and the outlet vane angle are equal to the impeller outlet average flow angle at the design point, the fluid of the outlet of the mixed-flow impeller on the shroud side is introduced into the guide vanes with almost no shock. Then, since the fluid introduced into the guide vanes is forcibly led by the guide vanes, the fluid will flow without separating from the wall surface of the shroud side and will reach the outlet portion of the guide vanes. Since at the outlet portion of the guide vanes, the vane angle is equal to the average flow angle of the design point, the flow angle of the fluid led by the guide vanes is equal to the flow angle of the fluid at the portion where guide vanes are not provided. Also, the curvature of the meridional plane flow path is small at the outlet of the guide vanes so that the meridional plane flow velocity is also kept substantially uniform in the widthwise direction. After all, the flow is kept constant in the width-wise direction.
Embodiments of the invention are discussed below in more detail by means of the accompanying drawings:

Fig. 1: is a longitudinal sectional view showing one embodiment of the invention;
Fig. 2: is a cross-sectional view taken along the line II-II of Fig. 1;
Fig. 3: is a graph showing a meridional plane velocity distribution in the widthwise direction at the end of a curved portion of the diffuser shown in Fig. 1;
Fig. 4: is a graph showing a comparison in adiabatic efficiency ratio between the mixed-flow compressor according to the embodiment shown in Fig. 1 and a conventional mixed-flow compressor;
Fig. 5: is an assemblage illustration of the compressor shown in Fig. 1;
Fig. 6: is a longitudinal sectional view showing another embodiment of the invention;
Fig. 7: is a longitudinal sectional view showing still another embodiment of the invention;
Fig. 8: is a cross-sectional view taken along the line VIII-VIII of Fig. 7; and
Fig. 9: is a longitudinal sectional view showing still another embodiment of the invention.

Fig. 1 is a longitudinal sectional view showing a mixed-flow compressor in accordance with an embodiment of the invention, in which a mixed-flow impeller 1 having a small curvature in a meridional flow path is fixed to a rotary shaft 2 by a nut 3. A pair of diffuser plates 4 and 5 each having a curvature in the vicinity of an outlet of the impeller 1 are provided outside of the impeller 1. The diffuser plates 4 and 5 of the outlet form a diffuser 6 which has a curvature in the vicinity of the impeller 1. One of the diffuser plates 4 is locted on a so-called shroud side, whereas the other of the diffuser plates 5 is located on a hub side. Guide vanes 7 are arranged in a circular row at the curved portion of the flow path surface of the diffuser plate 4. The guide vanes 7 are partially provided in the width-wise direction of the flow path and it is preferable that their height ranges between 20 to 50% of the flow path width. In addition, an inlet vane angle α₁ and an outlet vane angle α₂ are equal to an average flow angle of the outlet of the mixed-flow impeller 1 at a design point (i.e., an average value of an angle defined by a fluid absolute velocity of the impeller outlet at the design flow rate point with respect to a tangential direction (circumferential direction)). The reason why the height of the guide vanes is in the range of 20 to 50% is that the effect preventing reverse flow at the curved portion would be eliminated at the curved portion below 20% and the incidence (or shock) loss at the off-design flow rate point (i.e., a loss generated due to the difference between the flow angle and the vane angle) is increased to lower the performance of the compressor above 50% (for example 100%).
A minimum inlet radius r_a of the guide vanes 7 is larger than a maximum outlet radius r_b of the impeller 1.
A casing 8 is provided radially outwardly of the diffuser plates 4 and 5 to define an outlet flow path 9. A suction pipe 10 is fixed on a gas suction side of the diffuser plate 4.
The operation of the mixed-flow compressor with the above-described arrangement for compressing gas will be described.
The gas is sucked into the impeller 1 through the suction pipe 10 and then gas is discharged into the diffuser 6 from the impeller 1. The gas flow is decelerated within the diffuser 6 and is introduced into the casing 8. In general, since the curvature of the meridional flow path of the mixed-flow impeller 1 is small, the flow at the outlet of the impeller 1 becomes uniform in the widthwise direction. Accordingly, the flow angle of the fluid on the side of the diffuser plate 4 at the outlet of the impeller 1 is substantially equal to the average flow angle in the widthwise direction, so that the fluid on the side of the diffuser plate 4 is introduced into the guide vanes 7 with almost no shock. Since the introduced fluid is forcibly guided by the guide vanes 7, the fluid may flow without separating away from the wall surface of the diffuser plate 4 and reach the outlet portion of the guide vanes 7. Since the curvature of the meridional plane flow path is small at the outlet portion of the guide vanes 7, i.e., the terminal portion of the curvature, the flow is forcibly led by the guide vanes 7 (whose height is 40% of the the diffuser 6) and becomes uniform in the widthwise direction as shown in Fig. 3.
Fig. 4 shows the specific advantage according to this embodiment and the adiabatic efficiency ratio between a conventional mixed-flow compressor using the radially curved diffuser without any vanes and the compressor according to the present embodiment. Curve F indicates the adiabatic efficiency ratio at each suction flow rate of the conventional mixed-flow compressor, and curve E indicates the adiabatic efficiency ratio at each suction flow rate of the mixed-flow compressor according to the present embodiment. The reference value is defined by regarding as 1.0 the maximum value of the adiabatic efficiency of the mixed-flow compressor according to the present embodiment. As is apparent from Fig. 4, it is possible to considerably improve the adiabatic efficiency ratio in comparison with the conventional mixed-flow compressor having the diffuser without vanes.
As desribed above, since according to the present embodiment it is possible to prevent the separation of the flow at the curved portion of the diffuser, it is possible to considerably reduce the loss at the curved portion and to make uniform the flow in the widthwise direction at the outlet portion of the guide vanes, thereby largely enhancing the performance of the diffuser after the outlet portion of the guide vanes. In addition, since the meridional plane flow path of the diffuser is curved in the radial direction, the length of the flow path may be reduced in comparison with that of the conventional mixed-flow diffuser and the frictional loss may also be reduced. As a result, the performance of the mixed-flow compressor may be largely enhanced in comparison with the conventional compressor. Furthermore, since the rotary shaft of the compressor may be shortened, the critical speed of the rotor may be increased.
Fig. 5 is an illustration of the assemblage of the compressor of the embodiment shown in Fig. 1. First of all, the mixed-flow impeller 1 is fitted with the rotary shaft 2 by moving the impeller in the axial direction as indicated by the arrow A. Then, the impeller 1 is fastened to the rotary shaft 2 by the nut 3. The casing 8 integral with the diffuser plate 4 on which the guide vanes 7 are mounted is moved in the axial direction as indicated by the arrow B and is inserted into a fit portion 12 of the diffuser plate 5 which has been coupled with the rotary shaft 2 through bearings. In this embodiment, since the minimum inlet diameter of the guide vanes 7 is larger than the maximum outlet diameter of the impeller 1, it is also advantageously easy to assemble the compressor.
Fig. 6 is a longitudinal view showing another embodiment. In this embodiment, in the same way as Fig. 1, the diffuser 6 is composed of a pair of diffuser plates 4 and 5 each having a curvature in the meridional plane and guide vanes 7 arranged in a circular row at the curved portion on the flow path surface of the diffuser plate 4. The inlet angle and the outlet angle of the guide vanes 7 are substantially equal to an impeller outlet average flow angle at the design point. Also, for the same reason as that in Fig. 1, the height of the guide vanes 7 ranges between 20 to 50% of the flow path width. Then, the inlet radius r_a of the guide vanes 7 is larger than the outlet maximum radius r_b of the impeller 1 and is kept constant.
Also in this mixed-flow compressor, in the same manner as in Fig. 1, the fluid on the side of the diffuser plate 4 at the outlet of the mixed-flow impeller 1 is led by the guide vanes without separating away from the wall surface and reaches the outlet of the curved portion. The flow is kept substantially constant in the widthwise direction at the outlet of the curved portion. Accordingly, in the same manner as in Fig. 1, the performance of the diffuser 6 is considerably enhanced. Furthermore, since the meridional plane flow path is curved in the radial direction, the axial length of the compressor is shortened. Therefore, also in this embodiment, it is possible to make the mixed-flow compressor small in size and it is also possible to increase the critical speed of the rotor.
Furthermore, since the inlet radius of the guide vanes 7 is larger than the maximum outlet radius of the impeller 1 and kept constant, it is possible to facilitate the assemblage of the compressor and easier to manufacture the diffuser than in the case of Fig. 1.
Fig. 7 is a longitudinal sectional view showing still another embodiment. Fig. 8 is a cross-sectional view taken along the line VIII-VIII of Fig. 7. In this embodiment, in the same way as in Fig. 1, the diffuser 6 is composed of a pair of diffuser plates 4 and 5 each having a curvature in the meridional plane and guide vanes 11 arranged in a circular row on the flow path surface of the diffuser plate 4. The guide vanes 11 are provided not only on the curved portion of the flow path surface of the diffuser plate 4 but also on the parallel portion downstream of the curved portion. The inlet and outlet angles are substantially equal to the impeller outlet average flow angle at the design point. Also, for the same reason as that of Fig. 1, the height of the guide vanes 11 ranges between 20 to 50% of the flow path width. The inlet radius r_a of the guide vanes 11 is larger than the outlet maximum radius r_b of the impeller 1 and is kept constant.
Also in this mixed-flow compressor, in the same manner as in Fig. 1, the fluid on the side of the diffuser plate 4 at the outlet of the mixed-flow impeller 1 is led by the guide vanes 11 without separating away from the flow path surface and reaches the outlet of the curved portion. At the outlet of the curved portion, the flow is made substantially uniform in the widthwise direction. However, in the case where no guide vanes 11 are provided at the parallel portion downstream of the curved portion, it is possible that the distortion of inlet flow is increased toward the downstream side. Accordingly, since the guide vanes 11 are provided to extend to the parallel portion to thereby keep uniform the flow along the parallel portion, it is also possible to enhance the diffuser performance, i.e., the performance of the mixed-flow compressor in comparison with the case shown in Fig. 1.
Incidentally, in this embodiment, since the meridional flow path of the diffuser 6 is curved in the radial direction, the axial length of the compressor may be reduced. It is therefore possible to make small the mixed-flow compressor and to increase the critical speed of the rotor also in this embodiment.
Furthermore, since the inlet radius of the guide vanes 11 is larger than the outlet maximum radius of the impeller 1 in the same manner as in Fig. 6, it is possible to facilitate the assemblage of the compressor according to this embodiment, and it is possible to facilitate the manufacture of the diffuser in comparison with the case of Fig. 1.
Fig. 9 is a longitudinal sectional view according to still another embodiment. In this embodiment, the diffuser 6 is composed of a diffuser plate 5 having a curvature in the meridional plane, a diffuser plate 4 bent in the radial direction immediately after the inlet thereof, and guide vanes 11 arranged in a circular row on the flow path surface of the diffuser plate 4. The guide vanes 11 are provided at a section between the inlet and outlet of the diffuser 6. The inlet and outlet angles are substantially the same as the impeller outlet average flow angle at the design point. Also, for the same reason as that of Fig. 1, the height of the guide vanes 11 is in the range of 20 to 50% of the flow path width. The inlet radius r_a of the guide vanes 11 is larger than the outlet maximum radius r_b of the impeller 1 and is kept constant in the widthwise direction.
Also, in this mixed-flow compressor, in the same manner as in Fig. 1, since the fluid on the side of the diffuser plate 4 at the outlet of the mixed-flow impeller 1 is led by the guide vanes 11 without separating away from the flow path surfaces, the flow within the diffuser is kept substantially uniform in the widthwise direction. Accordingly, the performance of the diffuser 6 is largely improved. Furthermore, since the meridional plane flow path of the diffuser 6 is curved in the radial direction immediately after the inlet, the axial length of the compressor is shorter than that shown in Fig. 1. Accordingly, it is possible to make smaller the mixed-flow compressor and to increase the critical speed of the rotor in this embodiment.
In addition, since the inlet radius of the guide vanes 11 is larger than the outlet maximum radius of the impeller 1 and is kept constant, it is possible to facilitate the assemblage of the compressor and to simplify the manufacture of the diffuser in comparison with the case of Fig. 1.

Claims

A mixed-flow compressor comprising
- a mixed-flow impeller (1), in which the flow path in the meridional plane of the impeller (1) is slanted relative to a radial direction, and

- a pair of diffuser plates (4, 5) provided downstream of the mixed-flow impeller (1), such that the inlet portion of the flow path through said diffuser plates (4, 5) is substantially aligned with the outlet flow path of said mixed-flow impeller (1) and the outlet portion of the flow path through said diffuser plates (4, 5) is directed in a radial direction, and

- guide vanes (7) arranged on one of the diffuser plates (4) circumferentially around the rotary axis of the impeller in a circular row,
characterized in that
- the diffuser flow path is curved in a meridional plane in the vicinity of the outlet of said mixed-flow impeller (1), and

- the guide vanes (7) are arranged on the diffuser plate (4) on the shroud side such that in the radial direction of the impeller (1) the minimum inlet radius (r_a) of the guide vanes (7) is larger than the maximum radius (r_b) of said mixed-flow impeller (1),

- the height of the guide vanes (7) in the axial direction of the impeller (1) being less than the distance between the diffuser plates (4, 5).
A mixed-flow compressor according to claim 1, characterized in that the guides vanes (7) extend to the outlet portion of said flow path through the diffuser plates (4, 5)
The compressor according to claim 1 or 2, characterized in that the inlet angle (α₁) of said guide vanes (7, 11) relative to a tangent of a circle connecting the respective upstream ends of said guide vanes (7, 11) and the outlet angle (α₂) of said guide vanes (7, 11) relative to a tangent of a circle connecting the respective downstream ends of said guide vanes (7, 11) are substantially equal.
The compressor according to one of the preceding claims, characterized in that the height of said guide vanes (7, 11) in the axial direction of the impeller (1) ranges between 20 to 50 % of the width of the flow path.